Conclusions regarding the consequences of climate change for the agriculture
sector in the SAR (Reilly et al., 1996) provide an important benchmark
for this section. The focus in this section is on basic mechanisms and processes
that regulate the sensitivity of agriculture to climate change, relying mostly
on research results since the SAR. Specifically, we ask how the conclusions
of the SAR have stood the test of new research. Research advances since the
SAR have brought several new issues to lightfor example, understanding
the adaptation of agriculture to climate change.

The discussion in this section is guided by the State-Pressure-Impact-Response-Adaptation
model (see Figure 5-1). The pace of social, economic,
and technological change in the agriculture sector will steadily transform the
setting in which climate change is likely to interact with sensitive features
of the food system. The current state of the sector and important trends that
would transform it provide a baseline against which to examine the potential
consequences of climate change (Section 5.3.1). Multiple
pressures are being exerted on the agriculture sector, including the need to
meet rising demand for food and fiber, resource degradation, and a variety of
environmental changes (Section 5.3.2). Agricultural impacts,
response, and adaptation are discussed concurrently because they are inseparable
parts of the calculus of the vulnerability of agricultural systems to climate
change. Hence, we consider the response and adaptive potential of agriculture
in each of the succeeding sections. Agriculture is likely to respond initially
to climate change through a series of automatic mechanisms. Some of these mechanisms
are biological; others are routine adjustments by farmers and markets. Note
that we equate response with automatic adaptation, as discussed in Chapter
18.

Climate change will impact agriculture by causing damage and gain at scales
ranging from individual plants or animals to global trade networks. At the plant
or field scale, climate change is likely to interact with rising CO2
concentrations and other environmental changes to affect crop and animal physiology
(Section 5.3.3). Impacts and adaptation (agronomic
and economic) are likely to extend to the farm and surrounding regional scales
(Section 5.3.4). Important new work also models agricultural
impacts and adaptation in a global economy (Section 5.3.5).
Finally, the vulnerabilities of the agriculture sector, which persist after
taking account of adaptation, are assessed (Section 5.3.6).

5.3.1. State of the Global Agricultural Sector

As Reilly et al. (1996) argue in the SAR, one of the foremost goals
for global agriculture in coming decades will be expansion of the global capacity
of food and fiber in step with expansion of global demand. Agriculture in the
20th century accomplished the remarkable achievement of increasing food supply
at a faster rate than growth in demand, despite rapidly growing populations
and per capita incomes. Key summary indicators of the balance between global
demand and supply are world prices for food and feed grains. Johnson (1999)
and Antle et al. (1999a) show that during the second half of the 20th
century, real (inflation-adjusted) prices of wheat and feed corn have declined
at an average annual rate of 1-3%. Climate change aside, several recent
studies (World Bank, 1993; Alexandratos, 1995; Rosegrant et al., 1995;
Antle et al., 1999a; Johnson, 1999) anticipate that aggregate food production
is likely to keep pace with demand, so that real food prices will be stable
or slowly declining during the first 2 decades of the 21st century.

According to the U.S. Department of Agriculture (1999), food security1
has improved globally, leading to a decline in the total number of people without
access to adequate food. The declining real price of food grains has greatly
improved the food security of the majority of the world's poor, who spend a
large share of their incomes on these staples. The global number, however, masks
variation in food security among regions, countries, and social groups that
are vulnerable because of low incomes or a lack of access to food (FAO, 1999a).
In lower income countries, political instability and inadequate physical and
financial resources are the root causes of the food security problem (see Section
5.3.6). In higher income, developing countries, food insecurity stems from
unequal distribution of food that results from wide disparities in purchasing
power.

Agricultural production and trade policies also affect global food availability
and food security. There is a widespread tendency for high-income countries
to maintain policies that effectively subsidize agricultural production, whereas
low-income countries generally have policies that tax or discourage agricultural
production (Schiff and Valdez, 1996). Many low-income countries also pursue
policies that promote food self-sufficiency. Although all of these policies
tend to reduce the efficiency of agricultural resource utilization in low- and
high-income countries, they have not changed long-run trends in global supply
and demand (Antle, 1996a).

Relatively few studies have attempted to predict likely paths for food demand
and supply beyond 2020. There are reasons for optimism that growth in food supply
is likely to continue apace with demand beyond 2020. For example, population
growth rates are projected to decline into the 21st century (Bos et al.,
1994; Lutz et al., 1996; United Nations, 1996), and multiple lines of
evidence suggest that agricultural productivity potential is likely to continue
to increase. Rosegrant and Ringler (1997) project that current and future expected
yields will remain below theoretical maximums for the foreseeable future, implying
opportunities for further productivity growth.

Other analysts are less optimistic about long-term world food prospects. For
example, there is evidence that the Asian rice monoculture may be reaching productivity
limits because of adverse impacts on soils and water (Pingali, 1994). Tweeten
(1998) argues that extrapolation of the downward trend in real food prices observed
in the latter half of the 20th century could be erroneous because the supply
of the best arable land is being exhausted and rates of productivity growth
are declining. At the same time, demand is likely to continue to grow at reasonably
high rates well into the 21st century. Other studies indicate concerns about
declining rates of investment in agricultural productivity and their impacts
on world food production in some major producing and consuming areas (Hayami
and Otsuka, 1994; Rozelle and Huang, 1999). Ruttan (1996) indicates that despite
advances in biotechnology, most yield improvements during the first decades
of the 21st century are likely to continue to come from conventional plant and
animal breeding techniques. These concerns about future productivity growth,
if correct, mean that simple extrapolation of yield for impact assessment (e.g.,
Alexandratos, 1995) may be overoptimistic. The implication is that confidence
in predictions of the world food demand and supply balance and price trends
beyond the early part of the 21st century is low.

Box 5-3. Impacts of Climate Change and Elevated CO2 on
Grain and Forage Quality from Experimentation

The importance of climate change impacts on grain and
forage quality emerges from new research. For rice, the amylose content
of the graina major determinant of cooking qualityis increased
under elevated CO2 (Conroy et al., 1994). Cooked rice
grain from plants grown in high-CO2 environments would be
firmer than that from today's plants. However, concentrations of
iron and zinc, which are important for human nutrition, would be lower
(Seneweera and Conroy, 1997). Moreover, the protein content of the grain
decreases under combined increases of temperature and CO2
(Ziska et al., 1997).

With wheat, elevated CO2 reduces the protein
content of grain and flour by 9-13% (Hocking and Meyer, 1991; Conroy
et al., 1994; Rogers et al., 1996a). Grain grown at high
CO2 produces poorer dough of lower extensibility and decreased
loaf volume (Blumentahl et al., 1996), but the physiochemical
properties of wheat starch during grain fill are not significantly modified
(Tester et al., 1995). Increases in daily average temperatures
above 30°C, even applied for periods of up to 3 days, tend to decrease
dough strength (Randall and Moss, 1990). Hence, for breadmaking, the
quality of flour produced from wheat grain developed at high temperatures
and in elevated CO2 degrades.

With high-quality grass species for ruminants, elevated CO2 and temperature
increase have only minor impacts on digestibility and fiber composition
of cut material (Akin et al., 1995; Soussana et al., 1997).
The large increase in water-soluble carbohydrates in elevated CO2
(Casella and Soussana, 1997) could lead to faster digestion in the rumen,
whereas declines in nitrogen concentration occurring mainly with C3
species (Owensby et al., 1994; Soussana et al., 1996;
Read et al., 1997) reduce the protein value of the forage. The
protein-to-energy ratio has been shown to be more critical in tropical
climates than in temperate countries (Leng, 1990). Livestock that graze
low protein-containing rangeland forage therefore may be more detrimentally
affected by increased C:N ratios than energy-limited livestock that
graze protein-rich pastures (Gregory et al., 1999). Basically,
lowering of the protein-to-energy ratio in forage could reduce the availability
of microbial protein to ruminants for growth and production, leading
to more inefficient utilization of the feed base and more waste, including
emissions of methane.